A coordinate measuring apparatus of the above kind is, for example, disclosed in European patent publication 0,252,212 and includes a portal-shaped mechanical assembly. This mechanical assembly includes a portal which extends over a measuring table on which the workpiece to be measured is mounted. The portal can be driven by a corresponding drive in a first coordinate direction relative to the measurement table. The portal-shaped mechanical assembly additionally includes a transverse carriage in the region of the portal spanning the measurement table. The transverse carriage can be moved by a further drive along the spanned region of the portal in a second coordinate direction. Within the transverse carriage, a probe shaft is, in turn, movable in a vertical direction (that is, the third coordinate direction) by a third drive. A probe unit is provided at the end of the probe shaft for contacting a workpiece placed on the measurement or workpiece table.
A control element is provided for controlling the coordinate measuring apparatus. With this control element, the corresponding drives in the mechanical assembly shown can be controlled so that, in this way, the probe unit can be moved in all three coordinate directions.
The coordinate measuring apparatus shown has the following special features. The return force of the control element is developed by springs when the control element is deflected from its rest position. The springs pull the control element back to its rest position and the return force is always proportional to the deflected path. In this way, it is possible for the operator to deflect the control element to any extent desired. For the control element shown, the corresponding speed of the drive for the mechanical assembly in the particular coordinate measuring apparatus is usually pregiven proportional to the deflection from its rest position. For this reason, it is easily possible for the operator to generate high startup movement accelerations or braking decelerations which can lead to a rapid wear of the mechanical assembly and even to a destruction thereof.
Another characteristic of the coordinate measuring apparatus is that the probe can be destroyed when contacting a workpiece when utilizing very fine probe pins. This is so because the operator can effortlessly actuate the control element without realizing the forces acting on the probe.
In other measuring tasks, it is often required that the operator move the above-mentioned mechanical assembly in several axes simultaneously in order to move the probe unit, for example, along a scanning line arranged so as to be inclined in space. A precise control of the probe unit along this spatially-inclined direction is, however, virtually impossible with the control element of the above coordinate measuring apparatus.